This invention relates generally to apparatus for preparing matrices having frangible particulate material. More particularly, this invention concerns an easily cleaned, double acting metering pump asembly for supplying a molten matrix containing frangible particulate material to a molten parent matrix for mixing in a static mixing device.
The automobile industry makes extensive use of metallic paints having frangible particles of aluminum, for example, which exhibit a high luster when applied as a surface finish. These metallic paints are often applied as a powder in a spraying operation and cured by subsequent exposure to heat.
In the preparation of metallic paints, the aluminum particulate material has, in the past, been extruded along with the parent matrix so that a homogeneous dispersion of the particulate material results. The extrusion operation, however, exerts high shear forces on the relatively small frangible particles. Typically the particles are planar flakes that exhibit the desired reflective properties. The high shear forces have caused the planar particulate material to be bent, torn and otherwise damaged so that when the resulting metallic paint is applied to a surface an undesirable proportion of spurious reflections is obtained and the lustrous properties of the finish are deleteriously affected.
It has been found, as disclosed in the copending application noted above, that separation of the mixing and the extrusion processes can lead to markedly improved properties in the final product. The referenced application discloses an intermittently operable metering pump for supplying a molten matrix containing the frangible particulate material to a static mixing device wherein the frangible particulate material is homogeneously mixed with a flow of molten parent matrix from an extruding apparatus. The intermittent operation of the disclosed application is undesirable since it requires periodic charging of the metering apparatus and the concommitant delay.
Another persistent problem has been the effect of a metering pump on the frangible particles where mixing is not done in the extruder. Often, a gear pump is used which exposes an undesirably high portion of the particles to high shear forces between the gears and between the gears and the pump housing.
Accordingly, the need continues to exist for metering apparatus which is capable of overcoming the intermittent deficiencies of previous devices while avoiding the deleterious influence of high shear forces on frangible planar particulate material.